Chemical mechanical polishing (CMP) is a semiconductor wafer flattening and polishing process that combines chemical removal with mechanical buffing. It is used for polishing and flattening wafers after crystal growing, and for wafer planarization during the wafer fabrication process. CMP is a favored process because it can achieve global planarization across the entire wafer surface, can polish and remove all materials from the wafer, can work on multi-material surfaces, avoids the use of hazardous gasses, and is usually a low-cost process.
FIGS. 1A and 1B show an example effect of performing CMP. In FIG. 1A, a semiconductor wafer 102 has a patterned dielectric layer 104, over which a metal layer 106 has been deposited. The metal layer 106 has a rough top surface, and there is more metal than necessary. Therefore, CMP is performed, resulting in FIG. 1B. In FIG. 1B, the metal layer 106 has been polished down so that it only fills the gaps within the dielectric layer 104.
FIG. 2 shows an example CMP system 200 for polishing the wafer 102 of FIGS. 1A and 1B. The wafer 102, with its dielectric layer 104 and metal layer 106, is placed on a platen 202 connected to a rotatable rod 206. A polishing pad 204 is lowered over the wafer 102, specifically over the metal layer 106 thereof. The polishing pad 204 is also connected to a rotatable rod 206. Slurry 210 is introduced between the polishing pad 204 and the metal layer 106, and the polishing pad 204 is lowered, pressured against the metal layer 106, and rotated to polish away the excess, undesired metal from the metal layer 106. The platen 202 is rotated as in the opposite direction. The combined actions of the two rotations and the abrasive slurry 210 polish the wafer surface.
The polishing pad 204 can be made of cast polyurethane foam with fillers, polyurethane impregnated felts, or other materials with desired properties. Important pad properties include porosity, compressibility, and hardness. Porosity, usually measured as the specific gravity of the material, governs the pad's ability to deliver slurry in its pores and remove material with the pore walls. Compressibility and hardness relate to the pad's ability to conform to the initial surface irregularities. Generally, the harder the pad is, the more global the planarization is. Softer pads tend to contact both the high and low spots, causing non-planar polishing. Another approach is to use flexible polish heads that allow more conformity to the initial wafer surface.
The slurry 210 has a chemistry that is complex, due to its dual role. On the mechanical side, the slurry is carrying abrasives. Small pieces of silica are used for oxide polishing. Alumina is a standard for metals. Abrasive diameters are usually kept to 10–300 nanometers (nm) in size, to achieve polishing, as opposed to grinding, which uses larger diameter abrasives but causes more surface damage. On the chemical side, the etchant may be potassium hydroxide or ammonium hydroxide, for silicon or silicon dioxide, respectively. For metals such as copper, reactions usually start with an oxidation of the metal from the water in the slurry. Various additives may be found in slurries, to balance their ph, to establish wanted flow characteristics, and for other reasons.
FIG. 3 shows an example slurry mixing and delivery system 300 that may be used to mix and delivery the slurry 210 in-line to the CMP system 200 of FIG. 2. There are two intake pipes, a first pipe 302 and a second pipe 304. The first pipe 302 may be used to input abrasives, whereas the second pipe 304 may be used to input additives, such as surfactants and/or other additives. A surfactant is generally a substance capable of reducing the surface tension of a liquid in which it is dissolved. At the point 306 where the pipes 302 and 304 meet, the abrasives and the additives are mixed, such that there is a single flow of slurry in the connecting pipe 308. The pipe 308 ends in a nozzle 310 that outputs the slurry to the CMP system.
Slurry mixing, however, is vulnerable to certain problems. One problem is poor mixing of the abrasives and the additives. Optimally, the abrasives and the additives mix into a homogenous slurry mixture. However, at least occasionally the abrasives and the additives do not mix into a homogenous slurry mixture, which can result in non-optimal CMP to occur. Another problem is unstable slurry flow. If the slurry flow is not maintained at a steady and stable flow rate, or when the transient response of the slurry flow varies over time, non-optimal CMP can also occur. Currently, however, there is no adequate mechanism to conduct inline of monitoring the slurry mixing and delivery, to detect these and other problems.
Therefore, there is a need for in-line monitoring of slurry mixing and delivery. Such monitoring should be able to detect when the abrasives and the additives do not mix into a homogenous slurry mixture. Such monitoring should also be able to detect when there is an unstable slurry flow. For these and other reasons, there is a need for the present invention.